1. Field of the Invention
The present invention relates to a cap assembly and a secondary battery using the same, and more particularly, to a cap assembly having a vent which is capable of operating even when a low pressure is generated in a battery to thereby improve safety of the battery, and a secondary battery using the same.
2. Description of the Related Art
In recent years, active development and production of compact, lightweight portable electronic/electrical devices, such as cellular phones, notebook computers, and camcorders has been accompanied by widespread use of secondary batteries to power the portable electronic/electrical devices even in places without additional power sources. While dry cell batteries cannot be reused after they are electrically discharged once, the secondary batteries may be repeatedly used because they are capable of being charged and discharged.
Also, since it becomes possible to embody high-capacity secondary batteries, the secondary batteries may be applied not only to portable electronic devices but also to high-output products, such as hybrid automobiles and electric-powered tools.
Secondary batteries may include, for example, nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni—H) batteries, nickel-zinc (Ni—Zn) batteries, lithium ion secondary batteries, and lithium polymer secondary batteries. Among these secondary batteries, lithium ion secondary batteries are extensively employed because they operate at high voltages and have high energy density per unit weight.
Formation of lithium ion secondary batteries includes putting an electrode assembly and an electrolyte in an exterior material and sealing the exterior material. The lithium ion secondary batteries may be classified into can-type batteries and pouch-type batteries depending on the type of exterior material. Can-type lithium ion secondary batteries may be divided into cylinder-type batteries and prismatic-type batteries.
Formation of a cylindrical secondary battery includes forming an electrode assembly by stacking a positive electrode plate, a separator, and a negative electrode plate and winding the stack structure, putting the electrode assembly and an electrolyte into a cylindrical can, and sealing the cylindrical can using a cap assembly.
When a secondary battery is overcharged, the electrolyte may evaporate so that the resistance of the secondary battery can increase and heat can be generated. As a result, an internal temperature of the secondary battery may be raised, and an internal pressure of the secondary battery may also increase due to gas generated by an electrode assembly. This may result in accidents, such as fires and explosions.
In order to address the foregoing problems, a cylindrical secondary battery includes a safety device for interrupting current flow when an internal pressure of the battery is at a predetermined level or higher.
When the internal pressure of the secondary battery reaches the predetermined level or higher due to generation of a predetermined amount of gas in overcharge, a current breaker, such as a vent, may be enabled to prevent current flow, thereby providing battery safety.
However, a conventional vent operates under a relatively high pressure of about 9 kgf/cm2, meaning that current flow may be cut off only when an internal pressure of a secondary battery is about 9 kgf/cm2 or higher. Therefore, it is more difficult to ensure battery safety when the internal pressure of the secondary battery is lower.